1. Field of the Invention
The present invention relates to digital sample rate conversion and, more particularly, to image scaling using sample rate conversion.
2. Description of the Related Art
It is known that true scaling of images is computationally complex. Due to such complexities, conventional video graphics boards supporting scaling perform some simplified approach to scaling video images. Although the simplified approaches are significantly easier to implement, the resulting image quality is often unsatisfactory because the simplified approaches introduce undesired artifacts into the video images being scaled.
Conventional techniques for scaling video images primarily utilize either a pixel dropping technique or a linear interpolation technique. Both of these techniques are suitable for scaling images down but are not suitable for enlarging images.
In the pixel dropping technique, in order to scale down a video image, a digital sample value corresponding to a pixel will be periodically dropped. It is believed that most commercially available video graphics chips used in personal computers use the pixel dropping technique. A slightly improved approach to pixel dropping operates to low pass filter the incoming digital samples prior to pixel dropping. The low pass filtering operates to remove some of the high frequencies in the digital samples prior to the pixel dropping so as to reduce the undesirable artifacts. Commercially available personal computers from Apple Computer, Inc. of Cupertino, Calif., namely the PowerMac 7600 and 8500 series, included a video graphics board with an integrated circuit chip (Philips SAA7196) that was capable of scaling down video images used the pixel dropping approach with low pass filtering. In any event, both of these conventional techniques produce unwanted artifacts into the video images. These artifacts result from the introduction of high frequencies into the images as well as the introduction of phase errors.
Another conventional approach to scaling video images uses a linear interpolation technique. According to the linear interpolation approach, multipliers are used to calculate sample values that lie in arbitrary distance between two source sample values. The linear interpolation technique also introduces unwanted high frequencies and phase errors, but the extent to which they are introduced is reduced compared to the other previously mentioned conventional approaches due to the interpolation being performed.
Thus, there is a need for an improved technique for scaling images either up or down without introducing undesirable high frequencies or phase errors, so that the resulting scaled images are substantially free from artifacts.